marvin: update

- move usage documentation into its own document (timokau/marvin-mk2#17)
- react to commands in reviews (timokau/marvin-mk2#18)
- automatically set to `needs_work` on change requests (timokau/marvin-mk2#18)
- automatically set `needs_merge` -> `needs_review` on pushes (timokau/marvin-mk2#19)

nix/sources.json

@@ -5,10 +5,10 @@
         "homepage": "https://github.com/timokau/marvin-mk2",
         "owner": "timokau",
         "repo": "marvin-mk2",
-        "rev": "c26fb6e3b42bc31ffe954cd7f8ecfc14f7e9b26e",
-        "sha256": "0hmbwj0y7is5mjlk2r5ipphanqaq4yi83cqydd66pa0dzmi6idh9",
+        "rev": "9cba020608df52911e66dc7c534c5fbcd14471b5",
+        "sha256": "05cmlzz85c9sdr1cjilq82dp68bl4wppj3wlprvagfw5frcmy3nk",
         "type": "tarball",
-        "url": "https://github.com/timokau/marvin-mk2/archive/c26fb6e3b42bc31ffe954cd7f8ecfc14f7e9b26e.tar.gz",
+        "url": "https://github.com/timokau/marvin-mk2/archive/9cba020608df52911e66dc7c534c5fbcd14471b5.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs": {